ixgbe: Advance version to 4.2.1

With the addition of X550em_x SFP+ support, the driver is now
functionally equivalent to what will be the 4.2.1 driver when
released, so change the version to match.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>